Intracellular calcium (Ca2+) regulates or controls cell functions as diverse as proliferation, secretion, contraction, motility, metabolism, and gene expression. In the last ten years, scientists have discovered that many cell surface receptors initiate a cascade of events which result in the release of Ca2+ from intracellular stores, such as the endoplasmic reticulum. The unexpected dynamic behaviours of regenerative Ca2+ release, buffering, reuptake, and gating of Ca2+ entry from the extracellular space are only beginning to be studied and are still poorly understood. We propose to focus our research efforts on four related aspects of Ca2+ regulation. The four aims of the proposal are to l) define the initial steps in the receptor-dependent release of Ca2+ by specific G proteins; 2) measure the distribution and function of one Ca2+ release channel, the inositol (1,4,5) triphosphate (IP3) receptor, on intracellular stores. We have made the first direct recordings of this channel on intracellular membranes and will study its regulation in vivo. Exciting preliminary findings on the regulation of nuclear calcium will be extended; 3) better define the identities and functions of intracellular Ca2+ buffers; and 4) identify, clone, and examine the regulation of Ca2+ entry channel pathways that replete intracellular Ca2+ stores. The Xenopus laevis oocyte will be the focus of our studies. The oocyte has many advantages for study of Ca2+ regulation including ease of experimental manipulation and simplicity. Cloned keys proteins in Ca2+ regulation are highly homologous to mammalian cells. Finally, substantial evidence has accumulated that Ca2+ regulation in these cells closely follows that of many inexcitable mammalian cells.